1. Field of the Invention
The present invention relates to a color image forming apparatus for use in an electrophotographic electrostatic-type copying machine, printer, facsimile device, etc.
2. Discussion of the Background
FIG. 11 illustrates the structure of a color image forming apparatus of the tandem type in which the image forming sections are arranged along the conveying belt.
In FIG. 11, color image forming sections; 1Y, 1M, 1C, and 1K for forming the image of the respective different colors yellow Y, magenta M, cyan C and black K are arranged in a row along a conveying belt 3 for conveying a transfer paper 2. The conveying belt 3 is suspended on a rotating driving roller (not shown) for driving the belt 3, at one side thereof, and on the conveying rollers (driven rollers) 4 and 5 at another one side thereof. The conveying belt 3 is rotatably driven in the direction shown by an arrow by the action of the conveying rollers"" rotation.
A paper feeding tray 6 containing therein the transfer papers 2 is provided below the conveying belt 3. The transfer sheet situated on the uppermost position of the transfer papers 2 contained in the paper feeding tray 6 is fed at the time of forming the image. At this time, the transfer paper is electrostatically adhered onto the conveying belt 3. The transfer paper 2 thus adhered is conveyed to the first image forming section (yellow) 1Y and the yellow image formation is performed in the section 1Y. The first image forming section 1Y is constructed with a photosensitive drum 7Y, a charging unit (charger) 8Y, an exposing unit 9Y, a developing unit 10Y and a photosensitive body cleaner 11Y.
The surface of the photosensitive body drum 7Y, after being uniformly charged by the charging unit 8Y, is exposed by laser light 12Y corresponding to the yellow image by the exposing unit 9Y, and thereby an electrostatic latent image is formed on the surface of the photosensitive body drum 7Y. The electrostatic latent image thus formed is developed by the developing unit 10Y. In such a way, the yellow toner image is formed on the photosensitive drum 7Y. The toner image is transferred onto the transfer paper by the transferring unit 13Y at a position (transfer position) where the photosensitive drum 7Y is brought into contact with the transfer paper on the conveying belt 3. At this time, a single color image (yellow) is formed on the transfer paper. On the transfer-finishing photosensitive drum 7Y, the excess toner remaining on the surface of the drum is cleaned (removed) by the photosensitive body cleaner 11Y before the next image formation.
The transfer paper 2 transferred with the single color image (yellow) formed by the first image forming section 1Y is conveyed to the second image forming section (magenta) 1M by the conveying belt 3. The toner image (magenta) formed on the photosensitive drum 7M is superposedly transferred onto the transfer paper 2.
The transfer paper 2 is further conveyed in order to the third image forming section (cyan) 1C and the fourth image forming section (black) 1K, and toner images formed in the same way are transferred thereto. At this time, the color images are respectively formed thereon. The transfer paper 2 passing through the fourth image forming section 1K and having the color image formed thereon is separated (peeled off) from the conveying belt 3. After fixing the transfer paper 2 by use of the fixing unit 14, the fixed paper is discharged.
In the tandem type image forming apparatus as shown in FIG. 11, the exact alignment of the respective colors is very important. The components (factors) affecting positional displacement are mainly as follows:
(1) skew;
(2) resist displacement in the sub-scanning direction;
(3) unevenness of pitch in the sub-scanning direction;
(4) magnification rate error in the main scanning direction; and
(5) resist displacement in the main scanning direction.
FIG. 12 is a perspective view illustrating the area around the positional displacement detecting section disclosed in the published specification of Japanese Laid-open Patent Publication No. 10-198110. FIG. 13 is an enlarged view illustrating the positional displacement detecting section. FIG. 14 is an enlarged view illustrating a slit shown in FIG. 13. The detecting section is composed of a light emitting unit 15, a slit 16 and a light receiving unit 17. The detecting section detects a detection mark 18 for the positional displacement formed on the conveying belt 3. The detecting section is disposed at both sides of the belt 3 in the main scanning direction. The detection marks 18 correspond to each of respective positions. The slit is composed of openings (having a width xe2x80x9caxe2x80x9d and a length xe2x80x9cbxe2x80x9d) oriented for respectively detecting the parallel lines in the main scanning direction (hereinafter, called the xe2x80x9ctransverse linexe2x80x9d) and the inclined parallel lines inclined from the above lines (hereinafter, called the xe2x80x9coblique linexe2x80x9d).
FIG. 15 is an enlarged view showing the detection marks. The detection marks 18 are composed of the respective transverse and oblique lines for K, C, M and Y. The width of the respective lines is same as the width xe2x80x9caxe2x80x9d of the slit opening portion. The length thereof is longer than the length b of the slit opening portion. In this background art embodiment, the positional displacement detection marks 18 are arranged in the order of K-C-M-Y. The four transverse lines and the four oblique lines form a pair of lines groups.
Furthermore, plural pairs of toner marks of same color and same shape exist at positions separated by half the circumferential length of the photosensitive body. The respective toner mark couples exist at positions separated by multiples of the half circumferential length of the photosensitive body, and the plural toner mark couples of same color and same shape exist at positions separated by the half circumferential length of the photosensitive body. Those marks form a pattern in which one or more toner marks exist in the (positional) interval of the toner marks couples. In this embodiment, four sets of marks are formed in the length of one circumferential circle of the photosensitive body.
The positional interval xe2x80x9cdxe2x80x9d between the respective lines is set so as not to cause any inversion of the respective line rows order even though a positional displacement occurs. In the way mentioned above, the detection signal at the time when the line arrives at the opening portion of the slit 16 has a waveform of a regular mountain or valley, and therefore the line center can be precisely obtained.
Employing those transverse lines and oblique lines, the detected time difference between the respective lines and the left-and-right detection results are compared with each other based on the standard of the K transverse line in the respective couples of marks, and then the average of the four couples is calculated. In such the way, the detection error due to the unevenness of the photosensitive body""s rotation can be determined. Thereby, the compensation of the skew, the sub-scanning resist displacement, the main scanning resist displacement and the main scanning magnification error can be precisely performed.
FIG. 16 is a timing chart illustrating the timing at the time of compensating the timing of writing in the sub-scanning direction, and FIG. 17 is a timing chart illustrating the timing at the time of compensating the timing of writing in the main scanning direction.
Referring to FIG. 16 and FIG. 17, the compensation of the positional displacement amount disclosed in the published specifications of Japanese Patent Nos. 2642351 and 2765626 are described hereinafter.
In FIG. 16, the compensation resolution is assumed to be 1 dot. The image area signal in the sub-scanning direction (writing enabling signal) adjusts the writing with the timing of the synchronized detection signal. As a result of the mark detection and a calculation, if the position of writing one dot is required to be obtained early, the writing enabling signal is advanced by one synchronized detection signal as shown in FIG. 16.
In FIG. 17, the compensation resolution is assumed to be 1 dot. As to the image writing clock, the clock signals of precisely-set phase can be obtained for the respective lines by the trailing (falling) edge of the synchronized detection signal. The writing of the image is performed in synchronism with the clock signal. The image writing enabling signal in the main scanning direction is also obtained in synchronism with the clock signal.
As a result of the mark detection and calculation, if the position of writing one dot is required to be obtained early, the writing enabling signal is advanced by one clock pulse. Furthermore, when the mark detection, the calculation result, and the magnification in the main scanning direction are shifted from those of the standard color, the magnification can be changed by use of a device capable of changing the frequency by very small step, that is, a clock generator, etc.
Heretofore, the background art regarding the color image forming apparatus has been described. However, according to such background art, there exists no advantageous functional effect for improving the above-mentioned color image forming apparatus.
The present invention has been made in view of the above-discussed and other problems. A primary object of the present invention is to provide a color image forming apparatus improving the defects and troublesome matters of the background art.
In the case of performing compensation of various displacements of the respective colors with a predetermined resolution R, although the positional displacement amount between the standard color and one of the other four colors can be made within R/2 in the background-art method of calculation and compensation, when the adjustment of the position is performed between n colors (nxe2x89xa73), the positional displacement becomes R at most between the colors on some occasions.
The above problems can be solved by the present invention. Another object of the invention is to provide a full-color image forming apparatus capable of obtaining output of high image quality by suppressing the positional displacement of the respective colors.